YSZ Surface Research Articles

Interfacial effects in ultra-thin nickel deposits on yttria-stabilized zirconia

Abstract Ultra-thin films with 0.2 ML of Ni vapor-deposited in UHV near RT on yttria-stabilized-zirconia YSZ(1 0 0), were studied by X-ray photoelectron spectroscopy (XPS). Upon extensive annealing of the deposits in UHV between 480 and 850 K, it was found that there exists a critical temperature, below which the deposits tend to be fully oxidized via the substrate and above which they are stable in the metallic state. Nickel oxide formed on YSZ by either substrate or RT gas-phase oxidation is stable in UHV at least up to the critical temperature. This critical temperature appears to be sensitive to the history of the clean substrate treatments, as expressed by the O1s/Zr3d XPS intensity ratio, being higher than 850 K for the fully oxidized state of the clean YSZ surface and falling even below 635 K after repeated Ar+ sputtering cycles and extensive heating of the clean substrate in UHV at 870 K leading to an oxygen-deficient surface.

Atomic and Electronic Structures of Ni/YSZ(111) Interface

Thin Ni films were deposited on the (111) surface of YSZ at 1073 K by a pulsed laser deposition technique. The interfacial atomic structure of Ni/YSZ was investigated by high-resolution transmission electron microscopy (HRTEM). It was found that Ni was epitaxially oriented to the YSZ surface, and the following orientation relationship (OR) was observed: (111) N ι ∥ (111) Y S Z , [110] N i ∥ [110] Y S Z . Geometrical coherency of the Ni/YSZ system was also evaluated by the coincidence of reciprocal lattice points (CRLP) method. It was found that the most coherent OR predicted by CRLP method was (705) N i ∥ (111) Y S Z , [010] N i ∥ [110] Y S Z , which was not consistent with the experimentally observed OR. To understand the detailed atomic structure, HRTEM image simulations were performed. However, simulated images based on both O-terminated and Zr-terminated interface models were quite similar to the experimental image, and thus it was hard to determine which model is comparable with the actual interface only by the HRTEM image simulations. In order to clarify the termination layer at the interface, electronic structures of the Ni/YSZ interface were investigated by electron energy-loss spectroscopy. It was found that significant differences were observed in O-K edge spectra between the interface and the YSZ crystal interior, and the spectrum from the interface showed similar features to the reference spectrum of bulk NiO. This indicates that the Ni-O interaction occurs at the interface to terminate the oxygen {111} plane of YSZ at the Ni/YSZ interface. In addition, the density of Ni-O bonds across the interface in the experimental OR was larger than that in the most coherent OR predicted by CRLP method, which also suggests that the on-top Ni-O bonds stabilize the Ni/YSZ(111) interface.

The nature and binding strength of carbon adspecies formed during the equilibrium dissociative adsorption of CH 4 on Ni–YSZ cermet catalysts

The equilibrium dissociative adsorption of CH 4 was studied over Ni–YSZ cermet catalysts for a deeper insight regarding the nature and binding strength of generated carbon species on the Ni–YSZ surface. Three main carbon species were detected by the reaction of carbon adspecies with H 2 to produce CH 4 or with O 2 to produce CO and CO 2. Carbidic species (C c) are reactive with H 2 and O 2 at temperatures below 600 K while adsorbed carbon (C a) species in equilibrium with CH x species react with H 2 and O 2 above 600 K. Graphitic carbon layers (C g) are formed upon CH 4 adsorption above 700 K and its main characteristic is the absence of any reactivity with H 2, to form CH 4. The binding energy of C a species with respect to graphite decreases with increasing coverage ranging between 7.32±0.03 and 6.5±0.04 eV for the low (<0.2 ML) and high (≈1 ML) coverage, respectively. The presence of 1% wt of Mo either suppresses the formation of adsorbed graphitic layers which are not reactive with H 2 or enhances the reactivity of adsorbed hydrogen atoms toward CH 4 at temperatures higher than 800 K, thus revealing the positive effect of Mo in inhibiting the formation of adsorbed graphitic layers.

Features and mechanism of atomic oxygen radical anion emission from yttria-stabilized zirconia electrolyte

Anion and electron emissions from Y 2O 3-stabilized ZrO 2 (YSZ) electrolyte were investigated by time of flight (TOF) spectroscopy. The atomic oxygen radical anion (O −) is the only anion species emitted from the YSZ surface. The emission branch ratio between O – and electrons is close to one. The Arrehenius plots for both O – and electron emission, derived from the temperature dependences, exhibit double linear behavior. The apparent activation energies in the low-temperature region (<620 °C) are significantly higher than those in the high-temperature region (>620 °C). It was also found that the extraction field can enhance the O – emission. The maximum emission current density for O – is 3.5 nA/cm 2 in our investigated range. The observed O – and electron emissions are attributed to the dissociation of the intermediate anions, O 2–, present on the YSZ anode surface. The rate-determining step for the O – emission is believed to be the desorption process in the low-temperature region, and the electron desorption or the electrochemical reaction in the high-temperature region.

The interaction of ultrathin nickel films with yttria-stabilized zirconia ( [formula omitted

Increasing amounts of nickel, from 0.2 up to 5 equivalent monolayers, were vapour-deposited on a clean YSZ (1 0 0) surface at room temperature and then annealed at 600 K for up to 19 h. The nickel deposit was gradually oxidized at a rate depending on the deposited amount. For submonolayer coverages, it was completely converted into NiO within a few hours, whereas for multilayer quantities only a small fraction was oxidized. Carbon monoxide TPD experiments showed an intensity attenuation of the CO desorption peak, down to extinction for complete oxidation of the nickel surface, accompanied by a decrease in the TPD peak maximum temperature. This unexpected nickel oxidation upon prolonged heating was attributed to an interfacial interaction of the deposited Ni with mobile oxygen species present on YSZ at elevated temperature, due to its ionic conductor character.

Development of LSM/YSZ Composite Electrodes for Thin Film Solid Oxide Fuel Cells

The effects of cathodic current passage, (La,Sr)MnO 3 (LSM) stoichiometry and the surface treatment of YSZ electrolyte on the performance of LSM/YSZ composite cathode were investigated. The cathodic current passage had an enhancing/activation effect on the composite cathode made of stoichiometric LSM powder (LSM-A) and YSZ powder, while it had little effect on the composite cathode made of A-site sub-stoichiometric LSM powder (LSM-B) and YSZ powder. An A-site deficiency (∼0.1) is effective in inhibiting the formation of the resistive layer such as La 2 Zr 2 O 7 /SrZrO 3 between LSM and YSZ. Also, surface treatment of YSZ electrolyte had a major effect on the cathode performance. After polishing the YSZ surface, the measured interfacial resistance of LSM-B/YSZ (50:50 wt%) decreased to 0.27 ohm cm 2 from 0.50 ohm cm 2 on as-sintered YSZ surface at 800°C in air. The results showed that LSM/YSZ composite cathode performance can be significantly improved by using A-site deficient LSM and thorough surface treatment of YSZ electrolyte.

Comparative Investigation of Polarization Mechanism of Sputter Deposited Cermet Cathode Interlayers by Impedance-Spectra Time Relaxation Transform Technique

The influence of the structure of ultra-dispersed YSZ+Pt cermet layers on polarization resistance of SOFC cathode was investigated. The cermet layers were deposited by reactive magnetron sputtering with inclined or direct beams to the YSZ surface. The polarization resistance was determined from the impedance spectra of the electrochemical cell. There is a unique transform of imaginary part of impedance frequency distribution to time relaxation distribution of the polarization resistance. The transformation method is described. The data obtained were interpreted in terms of the cathode reaction microkinetic model. The time relaxation spectra peaks near 10 -1 s and 10 -3 s were attributed to oxygen adsorption and oxygen ion diffusion, respectively. The influence of deposition and heating conditions, and cathode electrical bias on the time relaxation spectra were considered. The results were compared with morphology features of samples under investigations.

The impact of YSZ surface steps on structure and morphology of La 2Zr 2O 7 islands growing on YSZ( [formula omitted]) surfaces by vapour–solid reaction

La 2Zr 2O 7 (LZO) islands of well-defined crystallographic orientation are growing on YSZ(1 0 0) single crystal surfaces, when the latter are exposed to La 2O 3 vapours at elevated temperature. The initial stage of this topotaxial vapour–solid reaction is investigated by HRTEM, AFM, and XRD, specifically considering the influence of steps on the YSZ(1 0 0) surface onto the LZO nucleation process. It is shown how this influence results in a particular structure and morphology of the LZO islands. Compared with the previously studied cases of LZO island formation on flat surface regions and at the rims of surface pits, new features are observed. These are discussed in terms of the preferred nucleation of the islands at step kinks, their specific growth kinetics under the influence of the step edges, and the partial relaxation of stresses arising in the growing islands by the deformation of the step edges.

Application of novel SIMS technique for imaging the active sites of oxygen reduction at the SOFC cathode/electrolyte interfaces

The oxygen reduction active sites were visualized around the O2/SOFC cathode/electrolyte triple phase boundaries (TPB) by the16O/18O exchange techniques and secondary ion mass spectrometry (SIMS) analysis. The higher18O concentration is observed on the cathode top surfaces (La0.9Sr0.1MnO3-mesh, Au-mesh, and Ag-porous), which suggested the promotion of oxygen adsorption and oxygen surface exchange at the cathode. The oxygen diffusion through the bulk of cathode occurred at the La0.9Sr0.1MnO3-mesh and the Ag-porous cathodes, not at the Au-mesh cathode. On the YSZ surfaces after removing the cathode, the active sites for oxygen incorporation were analyzed by SIMS. The active sites for oxygen incorporation were at the La0.9Sr0.1MnO3/YSZ interface as well as the TPB. On the other hand, the active sites for oxygen incorporation are limited to the TPB in the case of the Au-mesh removed YSZ surface. From the SIMS analysis, the expansion of the active sites for oxygen incorporation is less than a few μm from the TPB lines.

Fabrication and characterization of ultraviolet-emitting diodes composed of transparent p-n heterojunction, p-SrCu2O2 and n-ZnO

An ultraviolet light-emitting diode (UV-LED) was realized using a p-n heterojunction composed of the transparent oxide semiconductors p-SrCu2O2 and n-ZnO. A Ni/SrCu2O2/ZnO/ITO multilayered film was epitaxially grown on an extremely flat YSZ (111) surface by a pulsed-laser deposition technique. SrCu2O2 (112) was preferentially grown on ZnO (0001) at 350°C, while the preferential plane was changed into the (100) when the temperature was increased to 600 °C. The grown films were processed by conventional photolithography followed by reactive ion etching to fabricate heterojunction diodes. The resulting devices exhibited rectifying I-V characteristics inherent to p-n junctions. A relatively sharp electroluminescence band centered at 382 nm, attributed to transitions associated with exciton-exciton collision or electron-hole plasma in ZnO, was generated by applying a forward bias voltage greater than the turn-on voltage of 3 V. UV-LED performance characteristics such as threshold current and conversion efficiency improved with higher SrCu2O2 deposition temperatures. On the other hand, increased laser power density at 600 °C during deposition raised the incidence of insulating layer formation between the p and n layers, probably due to migration of K+ ions doped as an acceptor impurity. The resulting p-i-n diode emits broad luminescence centered at 500 nm for forward voltage greater than 14 V.

Transparent Conducting Indium-Tin-Oxide Thin Film with Extremely Flatted Surface

ABSTRACTIndium-tin-oxide films were grown hetero-epitaxially on YSZ surface at a substrate temperature of 900 °C, and their surface microstructures were observed by using atomic force microscopy. ITO films grown on (111) surface of YSZ exhibited very high crystal quality; full width at half maximum of out-of-plane rocking curve was 54 second. The ITO was grown spirally, with flat terraces and steps corresponding to (222) plane spacing of 0.29 nm. Oxygen pressure during film growth is another key factor to obtain atomically flat surfaced ITO thin film.

Fabrication of PbTiO3/Pt/Yttria Stabilized ZrO2 Heteroepitaxial Films on Si Substrate

Fabrication of PbTiO3/Pt/yttria-stabilized ZrO2(YSZ) heteroepitaxial films on Si substrate was investigated with the aim of applying them to electric devices such as integrated infrared sensors and ferroelectric random-access memories (FRAMs). First, a (100)YSZ buffer layer was epitaxially grown on a (100)Si substrate using reactive electron-beam evaporation. In this process, a metallic Zr film was deposited before YSZ deposition in order to prevent a Si substrate surface from oxidizing. Then, Pt film was sputtered on the buffer layer. A rough YSZ surface attained by slightly etching the Si substrate surface is found to be essential to obtain (100)-oriented Pt films, while (111)-oriented Pt films were obtained on a smooth YSZ surface. This indicates that we can control the orientation of Pt film by controlling the morphology of the YSZ surface. Lastly, a (001)-oriented PbTiO3 film was epitaxially grown on (100)Pt/(100)YSZ/(100)Si using metalorganic chemical vapor deposition (MOCVD).

Improvement of a Single‐Chamber Solid‐Oxide Fuel Cell and Evaluation of New Cell Designs

The performance of a single‐chamber solid‐oxide fuel cell (SOFC) made from an yttria‐stabilized zirconia solid electrolyte with a (GDC)‐containing Ni anode and a cathode was found to be significantly enhanced by the deposition of Mn, Ga, Cr, Ce, and Lu oxide layers on the YSZ surface. In particular, the deposition of the Mn oxide layer increased the maximum power density from 161 to in a mixture of methane and air having a volume ratio of methane to oxygen of 1/1 at a flow rate of (methane , oxygen , nitrogen ), and at an operating temperature of 950°C. This effect was the result of the promoted anodic and cathodic reactions. Two types of cell designs were examined for the single‐chamber SOFC; the two electrodes were deposited on opposite surfaces (A‐type cell) and on the same face (B‐type cell) of the solid electrolyte. The A‐type cell showed an increasing power density with decreasing thickness of the solid electrolyte. The maximum power density was at a solid electrolyte thickness of 0.3 mm. The B‐type cell showed an increased power density for a decreased gap between the two electrodes. The maximum power density was for a gap of 0.5 mm between the two electrodes. In addition, the long‐term stability of the single‐chamber SOFC was also studied and found to have a direct relationship with the carbon deposition on the GDC‐containing Ni electrode. © 2000 The Electrochemical Society. All rights reserved.

Oxygen Exchange on a Highly Oriented Thin Film Electrode

AbstractThe cathodic kinetic processes on a highly oriented LSCO thin film electrode supported on YSZ(100) surface were studied with a 3-probe ac impedance method under varying bias potential and annealing temperatures. Three distinctive features observed in the impedance spectra were assigned to contributions from the ionic conduction of the YSZ electrolyte, the ionic transfer at the LSCO/YSZ interface, and the oxygen exchange on the LSCO electrode surface. The changes of the three features with respect to the annealing history and bias potential were measured. The impedance data were analyzed using an equivalent circuit model: (RelCel)(RinterfaceQinterface)(RsurfCsurf).

Room Temperature Operation of UV-LED Composed of TCO Hetero p-n Junction, p-SrCu2O2/n-ZnO

AbstractRoom temperature operation of UV LED is realized for the first time using a hetero p-n junction composed of transparent conductive oxides, p-SrCu2O2 and n-ZnO. Ni/SrCu2O2/ZnO/ITO multi-layered film was epitaxially grown on an extremely flat YSZ (111) surface by a PLD. The grown films were processed by a conventional photolithography, followed by reactive ion etching to fabricate p-n junction diode. The resultant device exhibited rectifying I-V characteristics inherent to p-n junction whose turn-on voltage was about 1.5V. A relatively sharp electro-luminescence band centered at 382nm was generated when applying the forward bias voltage larger than the turn-on voltage of 3V. The red shift in the EL peak was noticed from that of photo-luminescence (377nm), which was most likely due to the difference in the excited state density between the emission processes. The EL band is attributed to transition in ZnO, probably to that associated with electron-hole plasma. The photo-voltage was also generated when the p-n junction was irradiated with UV light of which energy coincided with both exciton and band-to-band transitions in ZnO.

Ceria films on zirconia substrates: models for understanding oxygen-storage properties

The oxygen-storage properties of ceria in three-way automotive catalysts are promoted and stabilized by mixing with zirconia. In the present study, this promotion was investigated using model catalysts in which ceria films were vapor deposited onto α-Al 2O 3, polycrystalline ZrO 2, polycrystalline Y 2O 3-stabilized ZrO 2 (YSZ), and YSZ(1 0 0), (1 1 1), and (1 1 0) single crystals. Following deposition of Pd, both TPD of CO and steady-state CO-oxidation kinetics suggest that the ceria films on the zirconia-based substrates were much more easily reduced than films on α-Al 2O 3. Polycrystalline zirconia and YSZ and the YSZ single crystals were equally effective in promoting ceria reducibility. Structural studies of ceria on YSZ(1 0 0), using both TEM and EDSXD (energy-dispersive, surface X-ray diffraction), demonstrate that ceria forms ordered overlayers on YSZ(1 0 0), oriented with respect to the YSZ surface. The lattice parameter for ceria is decreased by only 0.6% compared to bulk CeO 2, but the coherence length suggests that the overlayer may have a high defect density. It is suggested that the structure-directing properties of zirconia are responsible for the enhanced properties of ceria–zirconia mixed oxides.

Effect of Mixed‐Conducting Interfacial Layers on Solid Oxide Fuel Cell Anode Performance

The interfacial resistance of Ni‐‐stabilized (Ni‐YSZ) anodes on YSZ electrolytes has been reduced by inserting thin interfacial layers of ‐doped YSZ (YZT) or ‐doped , (YDC). Impedance spectroscopy measurements at temperatures ranging from 600 to 750°C typically showed a high frequency arc (HFA) and a low frequency arc (LFA). The HFA was reduced by the addition of either interlayer, with a larger reduction for YDC. This presumably resulted from enhanced charge transfer due to the mixed conductivity and/or enhanced redox reaction rate of the interfacial layer. The LFA, which was apparently related to mass‐transport processes, grew with decreasing YSZ surface roughness and increasing interlayer thickness. The overall interfacial resistance was minimized for layer thicknesses of ∼0.5 μm. The lowest interfacial resistances in 97% , 0.13 Ω cm2 at 750°C, and 0.29 Ω cm2 at 600°C, were obtained with 0.5 μm thick YDC interfacial layers.

Characterization of ceria coated YSZ by a platinum point electrode in H 2-H 2O atmosphere

Electrode reaction was compared between single crystal YSZ and yttria doped ceria coated YSZ with a platinum point electrode in H 2-H 2O atmosphere at 1173 K. The thickness of the ceria layer was altered between 200 and 1000 nm. The ceria coating on the YSZ surface gave no effect on the electrode interfacial conductivity from AC impedance measurements. The electrode interfacial conductivity was either proportional to 1 4 or 1 2 powers of P(H 2), while it was independent of P(H 2O) both at the YSZ and the ceria coated YSZ. There was little difference in the, DC polarization curves between YSZ and ceria-200 nm coated YSZ, except that of the ceria-1000 nm coated YSZ. Hence, the surface catalytic activity does not affect the rate of the electrode reaction. The present results do not confirm the effect of high electronic and ionic conductivity of ceria.

Observation of defects and growth orientations of YBa 2Cu 3O x thin films with YSZ buffer layers on Si

We have investigated defects and in-plate orientations of YBa 2Cu 3O x thin films prepared by pulsed laser deposition (PLD) with YSZ as a buffer layer. The films showed c-axis oriented growth with the transition temperature T co up to 87 K. Several types of defects including thermally induced cracks, grain boundaries and outgrowths were observed by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The grain boundary provided a favorable path for crack propagation. The outgrowths nucleated on the YSZ surface grew with stoichiometric composition. According to X-ray diffraction (XRD) and HRTEM studies the YSZ buffer layer grew with the orientation relationship, YSZ〈110〉//Si〈110〉 and YSZ(001)//Si(001) up to the YBCO/YSZ interface. The superconducting YBCO films on top grew mainly with YBCO〈100〉//Si〈110〉 and YBCO(001)//Si(001), with some minor portions of YBCO〈110〉//Si〈110〉 and YBCO(001)//Si(001).

Improvement of Anode Performance by YSZ Surface Modification

Improvement of Anode Performance by YSZ Surface Modification